1. Field of the Invention
The present invention relates to a recording paper and an image recording method thereof. In particular, the invention relates to a recording paper using so-called plain paper without coating layers containing substantial quantity of pigment on either surface thereof, and an image recording method for ink-jet printing and electrophotographic printing using the same.
2. Description of the Related Art
An image forming machine using an ink-jet printing method has characteristics such as: ease of coloring, low energy consumption, low noise generation during recording; and the ability to suppress production costs to low levels. Due to such advantages, image forming machines using ink-jet printing methods have become widely used in offices in recent years, with increases in the number of machines which use ink-jet printing methods combined with electrophotographic image recording machines such as laser printers and copy machines.
Recording media (recording paper) such as so-called plain paper, coated paper and glossy paper, white film, and transparent film are used in image forming with ink-jet printing methods. Particularly when an image forming machine using ink-jet printing methods is employed in an image forming machine of an electrophotographic printing method, such as a laser printer or copier in an office or the like, printing is conducted most often on plain paper. Among these machines most use plain paper, on which an image can be formed easily, which is readily available and at a low cost.
Therefore, enhancing the suitability for recording of plain paper image forming with ink-jet printing methods is extremely important. However, there have been the following problems when printing is conducted on plain paper in conventional image forming with ink-jet printing methods.
(1) A so-called feathering phenomenon occurs whereby ink flows out along fibers of plain paper. Feathering significantly deteriorates image quality, particularly the quality of printed characters/letters.
(2) So-called plain paper usually contains a sizing agent on the surface for making the surface water-repellent. Consequently, absorption of ink is delayed, causing so-called inter-color bleeding (ICB) at portions where different colors contact with each other.
(3) Since absorption of ink is delayed due to the water repelling properties of plain paper surfaces, faces which contact with printed surfaces become dirty when printed documents are stacked.
(4) Since colorants in ink hardly stay on the surface of plain paper, the coloring nature of color ink is inadequate.
(5) Since colorants in inks permeate into plain paper, printed images can be seen through from the back of plain paper—rendering double-sided printing impossible.
Ink-jet printers have attempted to produce high printing speed, comparable to that of laser printers, in accordance with an expanded market for ink-jet printer in the office. However it has been very difficult to attain improved permeability (dryability) and image quality whilst also the ability to carry out double-sided printing.
In the light of these problems, methods of promoting coagulation and sedimentation of components contained in the ink by surface treating pape with cationic substances such as cation polymers, polyvalent metallic salts or the like are proposed. Of these methods, a method of adding a substance (usualy a polymer) to inks which reacts with a cationic substance applied to the surface of paper, thereby enhancing the viscosity of the ink applied thereto, is particularly useful for improving image quality. This is because the non-uniform spread-out rates of colorants contained in ink when ink is applied to paper surfaces can be controlled when high viscosity is obtained.
However, when the acid value of the polymer described above is large (equivalent to the amount of the anionic hydrophilic groups reacting with the cationic substance), the viscosity of the ink is raised excessively, the jetting performance of the ink jetted out from the recording head, or permeability of the ink into the paper may be damaged. On the other hand, when the acid value is small, the polymer is in a state of emulsion, and the increase in the viscosity of the ink is suppressed. Thus the jetting performance can be secured. However, since the number of reacting groups contributing to the acid value is few in the polymer, the reactivity with the cationic substance contained on the surface of the paper becomes low, and large improvements in image quality can not then be obtained.
On the other hand, in parallel to the improvements of ink composition described above, diversification in colorants has progressed in recent years, and, in addition to dyes widely used conventionally, pigments are being used more and more often in color inks. Thereby, situations where ink-jet printers using dye based inks and ink-jet printers using pigment based inks live together in the same office has come about.
When colorants are dyes and when colorants are pigments, generally the following respective methods are used effectively for raising image quality with recording papers designed for ink-jet printing.
When the colorant is a dye, a method is mentioned whereby the dye is reacted with a cationic substance contained on the surface of the paper, forming an insoluble dye complex, which is then physically trapped on the surface of the paper, is mentioned.
When the colorant is a pigment, a method is mentioned whereby a low molecule cationic substance contained on the surface of the paper rapidly elutes into the ink applied to the surface of the paper. This has the effect of rapidly raising the the concentration of electrolytes in the ink, coagulating the pigment as a pigmented colloid and fixing the pigment on the surface of the paper.
In order to obtain the full improvement effects in image quality with both dyes and pigments, it is necessary to include on the surface of the paper large amounts of both low molecule cations having fast elution, and polymers having good low molecule fixing performance.
However, a paper containing large amounts of substances having strong ionicity on the surface may overreact to surrounding environmental changes, and the electric resistivity of the paper may be reduced. Therefore, when such paper is used for image formation using electrophotographic printing methods in laser printers and the copy machines or the like, the paper has a bad influence on the transferability of toner. This means that such paper is not suitable for image formation using electrophotographic printing methods. Therefore, it is necessary to use a different paper for each printer having different image forming methods in the office (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 10-166713, 7-257017, 8-216498 and 10-100531).
When ink containing a component which reacts with cationic substances contained on the surface of the paper, to become insoluble, is used for image formation, when the ink is applied to the surface of the paper, huge coagulates containing the colorant are formed. This gives a significant improvement in image quality. However, in practice, components having sufficient reactivity to cationic substances are restricted to polymers having significant numbers of hydrophilic groups. Even if ink containing these components is used for image formation, when the ink is applied to the surface of the paper, the viscosity of the ink is increased. Thereby, not only fast drying properties needed for high-speed printing, but also ink jetting performance may suffer (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 9-176995 and 2002-96547).